1. Field of the Invention
The present invention relates to a cruise control system for a vehicle capable of controlling any deviation to a value as small as possible.
2. Description of the Related Art
Known in the prior art is a cruise control system for an automobile provided with an internal combustion engine, wherein the cruise control system has an actuator for operating an output control member of an engine, such as a throttle valve, in such a manner that the vehicle speed is controlled to a target value when the system is in a cruise mode. The cruise control system includes a sensor, for detecting an actual speed of the vehicle, and means are provided for detecting a deviation of an actual vehicle speed from a target speed. The actuator is, for example, a vacuum actuator which is controlled by feedback to reduce the deviation. This type of system suffers from a drawback in that the engine speed temporarily drops when the cruise mode operation is commenced because of a delay in the response of the actuator. When the control of the actuator is moved from the accelerator pedal operated by the operator to the actuator, the throttle valve is temporarily closed, since the actuator operates slowly. Due to this closure of the throttle valve, a temporary decrease in vehicle speed occurs.
In order to prevent this decrease in vehicle speed when the cruise mode operation is commenced, a system has been proposed for quick control of the stroke of the actuator to a position close to a target position for obtaining the target vehicle speed. In this system, a fixed level signal is sent to the actuator for a short period after the cruise mode has commenced, so that the actuator is moved to the target position at the maximum speed (see Japanese Unexamined Patent Publication (Kokai) No. 61-44033). In this prior patent, the actuator is quickly operated so that the temporary closure movement of the throttle valve is made as small as possible. Then the feedback control is commenced to control the vehicle speed to the target value. However, this improved system still has a drawback in that there is a certain time delay before realizing the target stroke of the actuator which will allow the opening of the throttle valve after the feedback control is commenced. As a result, the vehicle speed is still temporarily decreased after the cruise mode operation has commenced.